In communication systems it is often desirable to convert continuous wave or analog information to digital form prior to transmission. The advantage of transmitting information in digital form is that it can be transmitted on a single channel regardless of the frequency characteristics of its original form. Thus, where there is a plurality of analog information, sources of each of which might require a separate channel for transmission, conversion of it to digital form permits its transmission on a single channel with the attendant advantages of savings in modulation equipment.
Naturally, where there are more than one source of information to be transmitted, some means for time sharing the single channel of transmission must be provided. Thus, where there are a plurality of sources of information, each source is transmitted in time sequence under some type of multiplexer control. If the sources of information are continuous, then each may share the channel during time slots. Further, it may be desirable that certain ones of the sources of information have more time slots for transmission than others dependent on the type of information to be transmitted.
The present invention contemplates a multiplexing system where a plurality of sources of continuous wave information is constantly present at the inputs of a multiplexer. Under the control of a read only memory, each channel of the multiplexer is sampled in a sequence and at a rate under the control of the program in a read only memory. The analog information of each sampled multiplexer channel is converted to digital information and then introduced to the transmit stream. The word length, i.e., the number of bits put into the transmit stream is also controlled by the read only memory. Thus, control of the word length makes for more efficient transmission, i.e., more efficient use of the bandwidth. Since power required to transmit is a function of bandwidth and the word length, i.e., number of bits required to transmit certain types of information is less than others, selective compression of words prior to transmission aids in conservation of transmission energy.
The present invention also includes means to place information originating in digital form into the transmit data stream. Under read only memory control, time slots normally allotted to data from the multiplexer are assigned to the digital information whose word length is also controlled by the read only memory.